1. Field of the Invention
The present invention relates to a pixel circuit for current-driving light-emitting devices disposed at respective pixels. The present invention is also concerned with an active-matrix display apparatus having a matrix of such pixel circuits, for controlling currents supplied to light-emitting devices, such as organic EL devices, with insulated-gate field-effect transistors disposed in the respective pixel circuits.
2. Description of the Related Art
Image display apparatuses, such as liquid-crystal display apparatuses have a matrix of liquid-crystal pixels and control the intensity of light passing through or reflected by the pixels depending on image information to display an image represented by the image information. Organic EL display apparatuses having organic EL devices as pixels also operate similarly. Unlike liquid-crystal devices, the organic EL devices are self-luminous devices. Therefore, the organic EL devices display more visible images than the liquid-crystal devices, do not require backlight, and have a high response speed. The luminance level (gradation) of each light-emitting device can be controlled by a current flowing therethrough, and hence the organic EL display apparatuses are current-controlled whereas the liquid-crystal display apparatuses are voltage-controlled.
Like the liquid-crystal display apparatuses, the organic EL display apparatuses are classified into a passive-matrix drive type and an active-matrix drive type. Though the passive-matrix drive configuration is simple in structure, it poses difficulty in producing a large-size, high-definition display apparatus. Consequently, efforts are mainly directed to developing an active-matrix display apparatus. According to the active-matrix drive scheme, a current flowing through a light-emitting device in each pixel circuit is controlled by an active device (generally, a thin-film transistor or TFT) disposed in the pixel circuit. Active-matrix drive systems are disclosed in the following patent documents: Japanese Patent Laid-Open No. 2003-255856; Japanese Patent Laid-Open No. 2003-271095; Japanese Patent Laid-Open No. 2004-133240; Japanese Patent Laid-Open No. 2004-029791; Japanese Patent Laid-Open No. 2004-093682; and Japanese Patent Laid-Open No. Hei 10-214042.